All About Batteries, Part 5: Carbon Zinc Batteries

In this article, we focus on carbon zinc batteries, including their advantages, disadvantages, and chemistry.

As you may recall, a few weeks ago, Max Maxfield roped me into his ongoing robot project. This led to my writing this series of articles on the various battery technologies available to us. Along the way, in addition to the nitty-gritty technology details, I'm including tips and tricks on selecting the most appropriate battery technology for your application, along with tidbits of trivia and nuggets of knowledge, as Max would say. In this article, we consider alkaline batteries, but first...

Tip No. 5: Selecting batteries (continued from this post)
After computing the watt-hours at the load and determining the acceptability of the loss component, we could simply choose a battery with the rated watt-hours (or amp-hours times average voltage). But watch out for the manufacturer's ratings. Not all watt-hours or amp-hours are created equal.

When the manufacturer has discharge curves, you can use your load to find the run time (see Tip No. 4). However, some battery chemistries do not lend themselves to high-rate discharge, so it is more useful to calculate the watt-hours or amp-hours by measuring a smaller load over discharge spans such as five or 20 hours, instead of a single hour. Then the watt-hours or amp-hours are normalized to one hour, which artificially inflates the capacity, making the battery look like it could actually draw the listed watts or amps for one hour. As mentioned earlier, when the manufacturer doesn't specify this time span, the battery usually will be somewhat close to the listed rating when discharged over the course of an hour.

Let's consider an example, courtesy of the LC-R061R3P datasheet from Panasonic. A discharge test is run where the average discharge voltage six volts, the load is a constant current at 65 ma, and the discharge test is stopped at the end-of-life voltage of 5.3 V in 20 hours. The manufacturer lists the rated capacity as: 1.3 Ah (20-hour rate). Some manufacturers list the time as a C rate, such as C/20.

This rate is normalized, 0.065 amps x 20 hours = 1.3 Amp-hours, or 6 V x 1.3 Ah = 7.8 Wh. Note that this does not mean that we can draw 1.3 amps for one hour. In fact, the manufacturer may be nice enough to give us the actual one-hour draw. In this case, the measured one-hour capacity came out to be 0.85 Ah.

The carbon zinc battery
Carbon zinc (or zinc carbon) was originally a wet cell (called the Leclanche cell) and made with a packed block of manganese dioxide. A non-wet version patented by Carl Gassner that used zinc chloride instead of ammonium chloride became available in 1886. In 1898, Conrad Hubert and W. Lawrence formed the Eveready Co. to sell the cells and further develop the technology. Over the years, Eveready and other companies have made improvements, such as sealing technologies and purer materials.

Carbon zinc batteries are the least expensive of the primary batteries. They are useful for low-current or intermittent devices. They have a moderate shelf life and (these days) are fairly tolerant of abuse.

Specific energy: 36 Wh/kg

Energy density: 92 Wh/L

Specific power: 10-27 W/kg

Discharge efficiency: 50-60% (medium discharge)

Energy/consumer price: 3.2 Wh/dollar

Self-discharge rate: 0.32%/month (newest types, at four-year term)

Cycle durability: not applicable (primary battery)

Nominal cell voltage: 1.5 V

Cutoff voltage: 0.75-0.9 V per cell, loaded (see manufacturer)

Temperature range: -20 to +55°C

Chemistry
It is interesting to note that the carbon isn't actually part of the important chemical reaction. Graphite and carbon are used only to increase the cell's conductivity.

The following two figures are for the AA-size, Zn/MnO2, 1215 battery from Eveready.

Discharge curves using a constant-current load for various cutoff voltages.

Discharge curves for a constant-resistance load, instead of a constant-current load.

Energizer offers a nice application manual on its website. In my next column, we'll look at some more tips and tricks, and we will consider another battery technology. In the meantime, I welcome any questions or comments.

I remember the old days breaking open a carbon-zinc battery and messing around with the carbon rod and the zinc casing. I guess I always was going to be an engineer. Certainly my parents had no clue of the potential (pun unintended) dangers and never a word was spoken about it.

We used to plate our copper pennies with mercury to turn them silver. And I often used to rub a piece of asbestos to get the fibres off.

Some stuff we did know was more dangerous though. We used to wander up to the railway lines (my best friend's father worked on the railways) and place a coin on the track and stand back. Then we would rush up to retireve the disfigured coin.

All these memories from a simple battery. Maybe we should call it battery backup!

I guess we all did dumb stuff. I took out the carbon rod from a battery, then connected it to the big alligator clips of a 12v car-battery charger. It makes a very very thick white smoke for a few seconds, then glows orrange. Makes a good prank, when put into my dad's speaker cabinet. At least I thought so at the time...

Coins on the train, I had a good laugh. Me and my cousin did spent lots and lots of time at the railway side. We invented a 'coin-from-the-track'- removal installation. The problem was this: With fast trains, the coin jumped far far away never to be found again. But with slow heavy cargo trains, the coin was flatter, just flipped off the track due to the lower speed.

We had this enormous switch, 100m of cable and a small electromotor + battery very close aside the track (really! ;-)

The train came... "Oooh it is a fast one!". One of us flipped the switch and the coin was saved. Actually it was more fun to save the coin than otherwise. The installation turned out to be very successful. Somewhere we have a metal sigar box full of very very flat coins.

I think these funny fantasies made us to become engineers, I am certain about that.

An interesting if useless bit of information, it's actually a federal offense to damage a coin in that manner, although I have a vague recollection of doing it myself when I was about 6. Slightly more useful info, if you drop money in Thailand or Malaysia (not sure which) and stamp on it with your foot to stop it rolling away you can be jailed for placing your foot on the kings face (must be Tahiland, I don't think Malaysia has a king). I read about this happening to a German tourist back in the 80's

I doubt that US law would extend to what was Rhodesia, but I'm sure it was against the law there too. Hell everything was against the law- you couldn't warn anyone about a police speed trap up ahead because it was defeating the ends of justice.

Slightly more useful info, if you drop money in Thailand or Malaysia (not sure which) and stamp on it with your foot to stop it rolling away you can be jailed for placing your foot on the kings face (must be Tahiland, I don't think Malaysia has a king).

I once wrote a blog for the late lamented Microcontroller Central about an issue I nearly had insulting the King of Swaziland. I wonder if Max would be intersted in re-publishing?

I live in Oz and we have the same law regarding coins/currency as the US and I believe the UK is the same so I wouldn't be surprised if Zimbabwe's earlier government would have been any different. Re the Thai law there are probably only a handful of countries with something similar.

Not that I have ever done the 'train flat' coin thing, but someone I used to know said that if you tape the coin to the track, it won't fall off the track and gives it a chance to get squished by more wheels. It isn't illegal for the government to destroy its own coins. But the person actually doing it must have sanction, I imagine.